High-resolution retina imaging and eye aberration diagnostics using stochastic parallel perturbation gradient descent optimization adaptive optics

ABSTRACT

A method for compensating an optic aberration of an eye comprising the steps of applying to the eye a measuring light beam formed of incoherent light to provide an applied incoherent measuring light beam. An image quality metric is determined in accordance with the applied incoherent measuring light beam and the aberration is compensated in accordance with said image quality metric. A perturbation is applied to the image quality metric to provide a perturbed image quality metric and a determination is made whether a predetermined image quality is obtained in accordance with the perturbed image quality metric. An incoherent source light is transmitted from an incoherent light source to a mirror and redirected from the incoherent light source to the retina of the eye using the mirror in order to provide the applied incoherent measuring light beam.

FIELD OF THE INVENTION

The invention relates to a method and a system for high-resolutionretinal imaging, eye aberration compensation, and diagnostics based onadaptive optics with direct optimization of an image quality metricusing a stochastic parallel perturbative gradient descent technique.

BACKGROUND OF INVENTION

Adaptive optics is a promising technique for both diagnostics of opticalaberrations of the eye and substantially aberration-free high-resolutionimaging of the retina. In existing adaptive optics techniques adaptivecorrection is based on illumination of the retina by a collimated laserbeam to create a small size laser spot on the retina surface withconsequent measurement of phase aberrations of the wave scattered by theretina tissue. Correction of eye optical aberrations is then performedusing the conventional phase conjugation technique.

This traditional approach has several important drawbacks. One importantdrawback is the danger due to an invasive use of the laser beam focusedonto the retina. Other drawbacks include overall system complexity andthe high cost of the necessary adaptive optics elements such as awavefront sensor and wavefront reconstruction hardware. Moreimportantly, due to aberrations the laser beam spot size on the retinais not small enough to use it as a reference point-type light source andhence conjugation of the measured wavefront does not result in optimaloptical aberration correction. Additionally, the traditional approachcan produce a turbid image that can make performing an operation with amicroscope difficult.

One prior art method using a laser is taught in U.S. Pat. No. 6,095,651entitled “Method and Apparatus for Improving Vision and the Resolutionof Retinal Images”, issued to Williams, et al. on Aug. 1, 2000. InWilliams, et al. teaches a method and apparatus for improving resolutionof retinal images. In this method, a point source of light is producedon the retina by a laser beam. The source is reflected from the retinaand received at a lenslet array of a Hartman-Shack wavefront sensor.Thus, higher order aberrations of the eye can be measured and data canbe obtained for compensating the aberrations using a system including alaser. U.S. Pat. Nos. 5,777,719 and 5,949,521 provide essentially thesame teachings. While these references teach satisfactory methods forcompensating aberrations, there is some small risk of damaging theretina since these methods require applying laser beams to the retina.

U.S. Pat. No. 5,912,731, entitled “Hartmann-type Optical WavefrontSensor” issued to DeLong, et al. on Jun. 5, 1999 teaches an adaptiveoptics system using adjustable optical elements to compensate foraberrations in an optical beam. The aberrations may be caused, forexample, by propagation of the beam through the atmosphere. Theaberrated beam can be reflected from a deformable mirror having manysmall elements, each having an associated separate actuator.

Part of the reflected beam taught by DeLong can be split off anddirected to impinge on a sensor array which provides measurementsindicative of the wavefront distortion in the reflected beam. Thewavefront distortion measurements can then be fed back to the deformablemirror to provide continuous corrections by appropriately moving themirror elements. Configurations such as this, wherein the array of smalllenses as referred to as a lenslet array, can be referred to asShack-Hartmann wavefront sensors.

Additionally, DeLong teaches a wavefront sensor for use in measuringlocal phase tilt in two dimensions over an optical beam cross section,using only one lenslet arrangement and one camera sensor array. Themeasurements of DeLong are made with respect to first and secondorthogonal sets of grid lines intersecting at points of interestcorresponding to positions of optical device actuators. While thismethod does teach the way to correct aberrations in a non-laser lightsystem, it cannot be used in the case of >>>>>>>> where lasers arerequired.

U.S. Pat. No. 6,007,204 issued to Fahrenkrug, et al. entitled “CompactOcular Measuring System”, issued on Dec. 28, 1999, teaches a method fordetermining refractive aberrations of the eye. In the system taught byFahrenkrug, et al. a beam of light is focused at the back of the eye ofthe patient so that a return light path from the eye impinges upon asensor having a light detecting surface. A microoptics array is disposedbetween the sensor and the eye along the light path. The lenslets of themicrooptics array focus incremental portions of the outgoing wavefrontonto the light detecting surface so that the deviations and thepositions of the focused portions can be measured. A pair of conjugatelenses having differing focal lengths is also disposed along the lightpath between the eye and the microoptics array.

U.S. Pat. No. 6,019,472, issued to Koester, et al. entitled “ContactLens Element For Examination or Treatment of Ocular Tissues” issued onFeb. 1, 2000 teaches a multi-layered contact lens element including aplurality of lens elements wherein a first lens element has a recesscapable of holding a volume of liquid against a cornea of the eye. Amicroscope is connected to the contact lens element to assist in theexamination or treatment of ocular tissues.

U.S. Pat. No. 6,086,204, issued to Magnante entitled “Methods andDevices To Design and Fabricate Surfaces on Contact Lenses and OnCorneal Tissue That Correct the Eyes Optical Aberrations” on Jul. 11,2000. Magnante teaches a method for measuring the optical aberrations ofan eye either with or without a contact lens in place on the cornea. Amathematical analysis is performed on the optical aberrations of the eyeto design a modified shape for the original contact lens or cornea thatwill correct the optical aberrations. An aberration correcting surfaceis fabricated on the contact lense by a process that includes laserablation and thermal molding. The source of light can be coherent orincoherent.

U.S. Pat. No. 6,143,011, issued to Hood, et al. entitled “HydrokeratomeFor Refractive Surgery” issued on Nov. 7, 2000 teaches a high speedliquid jet for forming an ophthalmic incisions. The Hood, et al. systemis adapted for high precision positioning of the jet carrier. An airwaybeam may be provided by a collimated LED or laser diode. The laser beamcan be used to align the system.

U.S. Pat. No. 6,155,684, issued to Billie, et al. entitled “Method andApparatus for Precompensating The Refractive Properties of the Human EyeWith Adaptive Optical Feedback Control” issued on Dec. 5, 2000. Billie,et al. teaches a system for directing a beam of light through the eyeand reflecting the light from the retina. A lenslet array is used toobtain a digitized acuity map from the reflected light for generating asignal that programs an active mirror. In accordance with the signal theoptical paths of individuals beams in and the beam of light are made toappear to be substantially equal to each other. Thus, the incoming beamcan be precompensated to allow for the refractive aberrations of theeyes that are evidenced by the acuity map.

Additional methods for using adaptive optics to compensate foraberrations of the human eye are taught in J. Liang, D. Williams and D.Miller, “Supernormal Vision and High-Resolution Retinal Imaging ThroughAdaptive Optics,” J. Opt. Soc. Am. A, Vol. 14, No. 11, pp.2884-2891,1997 and F. Vargas-Martin, P. Prieto, and P. Artal,“Correction of the Aberrations in the Human Eye with a Liquid-CrystalSpatial Light Modulator: Limits to Performance,” J. Opt. Soc. Am. A,Vol. 15, No. 9, pp. 2552-2561, 1998. Additionally, J. Liang, B. Grimm,S. Goelz, and J. Bille, “Objective Measurement of Wave Aberrations ofthe Human Eye with the Use of a Hartmann-Shack Wave-Front Sensor,” J.Opt. Soc. Am. A, Vol. 11, No. 7, pp. 1949-1957,1994 teaches such a useof adaptive optics.

Furthermore, it is known in the art to use a PSPGD optimizationalgorithm in different applications. For example, see M. Vorontsov, andV. Sivokon, “Stochastic Parallel-Gradient-Descent Technique forHigh-Resolution Wave-Front Phase-Distortion Correction,” J. Opt. Soc.Am. A, Vol. 15, No. 10, pp. 2745-2758,1998. Also see M. Vorontsov, G.Carhart, and J. Ricklin, “Adaptive Phase-Distortion Correction Based onParallel Gradient-Descent Optimization,” Optics Letters, Vol. 22, No.12, pp. 907-909,1997.

SUMMARY

The present inventions deal with new methods of high-resolution imagingof the retina, and adaptive correction and diagnostics of eye opticalaberrations using adaptive optics techniques based on parallelstochastic perturbative gradient descent (PSPGD) optimization. Thismethod of optimization is also known as simultaneous perturbationstochastic approximation (SPSA) optimization. Compensation of opticalaberrations of the eye and improvement of retina image resolution can beaccomplished using an electronically controlled phase spatial lightmodulator (SLM) as a wavefront aberration correction interfaced with animaging sensor and a feedback controller that implements the PSPGDcontrol algorithm.

Examples of the electronically-controlled phase SLMs include a pixelizedliquid-crystal device, micro mechanical mirrorarray, and deformable,piston or tip-tilt mirrors. Wavefront sensing can be performed at theSLM and the wavefront aberration compensation is performed using retinaimage data obtained with an imaging camera (CCD, CMOS etc.) or with aspecially design very large scale integration (VLSI) imaging chip (VLSIimager). The retina imaging data are processed to obtain a signalcharacterizing the quality of the retinal image (image quality metric)used to control the wavefront correction and compensate the eyeaberrations.

The image quality computation can be performed externally using animaging sensor connected with a computer or internally directly on animaging chip. The image quality metric signal is used as an input signalfor the feedback controller. The controller computes control voltagesapplied to the wavefront aberration correction. The controller can beimplemented as a computer module, a field programmable gate array (FPGA)or a VLSI micro-electronic system performing computations required foroptimization of image quality metrics based on the PSPGD algorithm.

The use of the PSPGD optimization technique for adaptive compensation ofeye aberration provides considerable performance improvement if comparedwith the existing techniques for retina imaging and eye aberrationcompensation and diagnostics. The first advantage is that the PSPGDalgorithm does not require the use of laser illumination of the retinaand consequently significantly reduces the risk of retina damage causedby a focused coherent laser beam. A further advantage is that the PSPGDalgorithm does not require the use of a wavefront sensor or wavefrontaberration reconstruction computation. This makes the entire systemlow-cost and compact if compared with the existing adaptive opticssystems for retina imaging. Additionally, the PSPGD algorithm can beimplemented using a parallel analog, mix-mode analog-digital or paralleldigital controller because of its parallel nature. This significantlyspeeds up the operations of the PSPGD algorithm, providing continuousretina image improvement, eye aberration compensation and diagnostics.

Thus, in the adaptive correction technique of the present inventionneither laser illumination nor wavefront sensing are required. Opticalaberration correction is based on direct optimization of the quality ofa retina image obtained using a white light, incoherent, partiallycoherent imaging system. The novel imaging system includes amulti-electrode phase spatial light modulator, or an adaptive mirrorcontrolled with a computer or with a specially designed FPGA or VLSIsystem. The calculated image quality metric is optimized using aparallel stochastic gradient descent algorithm. The adaptive opticalsystem is used in order to compensate severe aberrations of the eye andthus provide a high-resolution image of the retina tissue and the eyeaberration diagnostic.

DESCRIPTION OF THE DRAWINGS

FIGS. 1A,B show a schematic representation of system suitable forpracticing the eye aberration correcting method of the presentinvention.

FIG. 2 shows a flow chart representation of control algorithm suitablefor use in the system of FIG. 1 when practicing the method of thepresent invention.

FIGS. 3A,B show images of an artificial retina before and aftercorrection of an aberration.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, there is shown a schematic representation ofthe aberration correcting system 10 of the present invention. In theaberration correcting system 10 a light beam from a white light source 1is redirected by a mirror 2 in order to cause it to enter an eye. Inaccordance with the present invention the white light beam from thelight source 1 can be any kind of incoherent light.

The light from the mirror 2 reaches the retina 4 of the eye andreflected light exits the eye to provide two light beams, one passing ineach direction, as indicated by arrow 3. The exiting light beam thenpasses through an SLM 5. The light beam from the SLM 5 enters an imagesensor 6. The image sensor 6 can be a charge coupled capacitor device orany other device capable of sensing and digitizing the light beam fromthe SLM 5.

The imaging sensor 6 can include an imaging chip for performing thecalculations required to determine an image quality metric. The imagequality metric can thus be computed on the imaging chip directly or itcan be calculated using a separate computational device/computer 7 thatcalculates the image quality metric of the retina image. It is the useof a digitized image in this manner that permits the use of anincoherent light rather than a coherent light within the aberrationcorrection correcting system 10.

The computational device 7 sends a measurement signal representative ofthe image quality metric to a controller 8. The controller 8 implementsa PSPGD algorithm by computing control voltages and applying thecomputed control voltages to the SLM 5. The PSPGD algorithm used by thecontroller 8 can be any conventional PSPGD algorithm known to those ofordinary skill in the art. In the preferred embodiment of the invention,the controller 8 continuously receives digital information about thequality of the image and continuously updates the control voltagesapplied to the SLM 5 until the quality of the retina image is optimized.

Referring now to FIGS. 2 and 3A, B there are shown a flow chartrepresentation of a portion of a PSPGD control algorithm 20 for use incooperation with the aberration correcting system 10 in order topractice the present invention as well as representations of thecorrected image, both before and after correction. In order to simplifythe drawing a single iterative step of the PSPGD control algorithm 20 isshown with a loop for repeating the single iterative step until thequality of the compensation is acceptable.

In step 25 of the PSPGD control algorithm 20 a measurement andcalculation of the image quality metric is performed. This step includesthe retinal image capture and the calculation of the image qualitymetric performed by the sensor 5 and the computational device 7 in theaberration correcting system 10. The image captured at the beginning ofthe operation of the PSPGD control algorithm 20 can be substantially asshown in FIG. 3A.

One can use any relevant entity as an image quality metric. For example,in one embodiment of the PSPGD control algorithm 20 the image qualitymetric can be the sharpness function. A sharpness function suitable foruse in the present invention can be defined as

J=∫|∇ ² I(x,y)|dxdy

where I (x,y) is the intensity distribution in the image, and ∇² is theLaplacian operator over the image. The Laplacian can be calculated byconvolving the image with a Laplacian kernel. The convolving can involvea special purpose VLSI microchip. Alternately, the convolving can beperformed using a computer that takes a picture of the image using adigital camera. In another embodiment different digital high-passfilters can be used rather than the Laplacian operator.

Additionally, a frequency distribution function can be used rather thana sharpness function when determining the image quality metric. The useof a frequency distribution function allows the system to distinguishtissues of different colors. This is useful where different kinds oftissue, for example, different tumors, have different colors. Locatingtumors in this manner also permits the invention to provide tumorlocation information, such as a grid location on a grid having apre-determined reference in order to assist in diagnosis and surgery. Italso permits the invention to provide tumor size and type information.Additionally, the use of a frequency distribution function permits asurgeon to determine which light frequencies are best for performingdiagnosis and surgery.

The image quality metric J can also be calculated either optically ordigitally using the expression introduced in:

J=∫|F{exp[iγI(x,y)]}|⁴ dxdy

where F is the Fourier transform operator and γ is a parameter that isdependent upon the dynamic range of the used image.

In step 30 of the PSPGD control algorithm 20 random perturbations in thevoltages applied to the SLM 5 electrodes are generated. The SLM 5 can bea liquid crystal membrane for modifying the light beam according to theelectrical signals from controller 8 in a manner well understood bythose skilled in the art.

In order to generate the perturbations for application to the electrodesfor the SLM 5 random numbers with any statistical properties can be usedas perturbations. For example, uncorrelated random coin flipperturbations having identical amplitudes |du_(j)|=p and the Bernoulliprobability distribution:

δu _(j) =±π, Pr(δu _(j)=+π)=0.5

for all j=1, . . . , N(N=the number of control channels) and iterationnumbers can be used. Note that Non-Bernoulli perturbations are alsoallowed in the PSPGD control algorithm 20.

In step 35 of the PSPGD control algorithm 20 a measurement of theperturbed image quality metric and a computation of the image qualityperturbation δJ^((m)) are performed. Following the determination of theperturbed image quality metric, the gradient estimations:

{tilde over (J)}′ _(j) ^((m)) =δJ ^((m))π_(j) ^((m))

are computed as shown in step 40.

The updated control voltages are then determined as shown in step 45.Therefore, a calculation of:

u _(j) ^((m+1)) =u _(j) ^((m)) −γδJ ^((m))π_(j) ^((m)) , j=1, . . . , N

is performed.

To further improve the accuracy of gradient estimation in the PSPGDcontrol algorithm 20 a two-sided perturbation can be used. In atwo-sided perturbation two measurements of the cost functionperturbations δJ⁺ and δJ⁻ are taken. The two measurements correspond tosequentially applied differential perturbations +δu_(j)/2 and −δu_(j)/2.It follows that:

δJ=δJ ⁺ −δJ ⁻

and

{tilde over (J)}′ _(J) =δJδu _(j),

which can produce a more accurate gradient estimate.

The process steps 25-45 of the PSPGD control algorithm 20 are repeatediteratively until the image quality metric has reached an acceptablelevel as determined in step 50. The choice of an acceptable level of theimage quality metric is a conventional one well known to those skilledin the art. As shown in step 55 the aberration is then corrected and animage of the retina can be taken. The image resulting from the operationof the PSPGD algorithm 20 can be as shown in FIG. 3B.

The eye aberration function φ(x,y) can be calculated from known voltagesapplied to wavefront correction {u_(j)} at the end of the iterativeoptimization process and known response functions of{S_(j)(x,y)}wavefront correction${\phi \left( {x,y} \right)} = {\sum\limits_{j = 1}^{N}\quad {u_{j}{{S_{j}\left( {x,y} \right)}.}}}$

The description herein will so fully illustrate my invention that othersmay, by applying current or future knowledge, adopt the same for useunder various conditions of service. For example, the invention may beused for the determination of spectacle, contact lens, intralocular lens(phakic and aphakic), intracorneal lens or rings, or other means ofrefractive alterations. When making lenses using the present invention,the best data obtained using the optimization features can betransmitted from computation device 7 to a conventional lens makingdevice 9. Lens making device 9 can be any conventional device for makinglenses according to such data since the parameters optimized by theinvention are the same parameters required for visual activity using thelens.

We claim:
 1. A method for compensating an optic aberration of an eye,comprising the steps of: (a) applying to said eye a measuring light beamformed of incoherent light to provide an applied incoherent measuringlight beam; (b) determining an image quality metric in accordance withsaid applied incoherent measuring light beam; and (c) compensating saidaberration in accordance with said image quality metric.
 2. The methodfor compensating an optic aberration of eye of claim 1, furthercomprising the steps of: (a) applying a perturbation to said imagequality metric to provide a perturbed image quality metric; and (b)determining whether a predetermined image quality is obtained inaccordance with said perturbed image quality metric.
 3. The method forcompensating optic aberrations of an eye of claim 1, further comprisingthe steps of: (a) transmitting incoherent source light from anincoherent light source to a mirror; and (b) redirecting said incoherentsource light from said incoherent light source to the retina of said eyeusing said mirror in order to provide said applied incoherent measuringlight beam.
 4. The method for compensating optic aberrations of a eye ofclaim 1, further comprising the steps of reflecting said appliedincoherent measuring light beam from said retina to provide a reflectedlight beam and applying said reflected light beam to a spatial lightmodulator.
 5. The method for compensating optic aberrations of an eye ofclaim 4, further comprising the steps of applying said reflected lightbeam from said spatial light modulator to an image sensor for sensingsaid reflected light and providing signals representative of saidreflected light.
 6. The method for compensating optic aberrations of aneye of claim 5, further comprising the step of determining said imagequality metric in accordance with said signals representative of saidreflected light.
 7. The method for compensating optic aberrations of aneye of claim 6, further comprising the step of determining said imagequality metric as: J=∫|F{exp[iγI(x,y)]}|⁴ dxdy where F is a Fouriertransform and γ is a parameter dependent upon a dynamic range of saidreflected light beam.
 8. The method for compensating optic aberrationsof an eye of claim 7, further comprising the step of optimizing saidcontrol voltage using a parallel stochastic perturbative gradientdescent algorithm.
 9. The method for compensating optic aberrations ofan eye of claim 8, further comprising the step of performing a wavefrontaberration reconstruction computation.
 10. The method for compensatingan optic aberration of an eye of claim 9, including a wavefront sensorwherein the step of compensating said aberration comprises the step ofcompensating a plurality of wavefronts.
 11. The method for compensatingoptic aberrations of an eye of claim 6, further comprising the step ofcomputing a control voltage in accordance with said image qualitymetric.
 12. The method for compensating an optic aberration of an eye ofclaim 11, further comprising the step of applying said control voltageto said spatial light modulator for modulating said reflected light inaccordance with said control voltage.
 13. The method for compensating anoptic aberration of an eye of claim 12, further comprising the step ofapplying a voltage perturbation to aid control voltage in accordancewith said perturbation applied to said image quality metric.
 14. Themethod for compensating an optic aberration of an eye of claim 13,wherein said voltage perturbation comprises a random voltageperturbation.
 15. The method for compensating an optic aberration of aneye of claim 14, further comprising a plurality of random voltageperturbations having identical peturbation amplitudes.
 16. The methodfor compensating an optic aberration of an eye of claim 14, wherein saidrandom voltage perturbation comprises a plurality of voltageperturbations having a Bernoulli probability distribution.
 17. Themethod for compensating an optic aberration of an eye of claim 14,wherein said random voltage perturbation comprises a plurality ofsequentially applied differential voltage perturbations +du_(j)/2 and−du_(j)/2.
 18. The method for compensating optic aberrations of an eyeof claim 4, wherein said image quality metric is a sharpness function.19. The method for compensating an optic aberration of an eye of claim18, wherein said sharpness function comprises: J=∫|∇ ² I(x,y)|dxdy andI(x,y) is an intensity distribution of the image quality metric and ∇²is a Laplacian operator over said image quality metric.
 20. The methodfor compensating an optic aberration of an eye of claim 1, furthercomprising the step of determining a refractive state of said eye inaccordance with said compensating.
 21. The method for compensating anoptic aberration of an eye of claim 1, further comprising the step ofaltering said refractive state of said eye in accordance with saidcompensating.
 22. The method for compensating an optic aberration of aneye of claim 1, further comprising the step of determining a diseasediagnosis of said eye in accordance with said compensating.
 23. Themethod for compensating an optic aberration of an eye of claim 1,further comprising the step of performing surgery on said eye inaccordance with said compensating.